Iran calls to find the killers of Fakhrizadeh 2:23
(CNN) -
Iran's top nuclear scientist was assassinated on Friday outside Tehran, once again putting the Middle East on edge and threatening further unrest in the region.
This is what you should know.
What happened?
Mohsen Fakhrizadeh, who is believed to be the mastermind behind Iran's controversial nuclear program, was in a car east of Tehran when he was shot and killed on Friday afternoon.
While there are conflicting reports on how the attack unfolded, most accounts agree that it was a sophisticated attack with gunfire and an explosion.
But at least one account offered even more dramatic claims.
The semi-official Fars news agency claimed on Sunday that Fakhrizadeh was traveling with his wife in a bulletproof car, alongside three security personnel vehicles, when he heard what sounded like bullets hitting a vehicle and got out of his car to find out what it happened.
Remote controlled machine gun shooting?
At that point, a remote-controlled machine gun opened fire from a Nissan stopped about 150 meters from Fakhrizadeh's car, Fars said.
The scientist was hit at least three times and the Nissan exploded after the attack.
Weapons experts say the claim is technically possible.
But experts agree that those behind the murder are unlikely to choose this method due to its high risk of failure.
Iranian news agencies have also offered contradictory versions of events.
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It is also possible that the Iranian authorities are trying to exaggerate the sophistication of the attack to downplay the blatant weakness of its security apparatus exposed by the killing in broad daylight of one of its top officials.
The assassination has left Iran feeling exposed and vulnerable.
LEE
: Iranian nuclear scientist was shot dead with a remote-controlled machine gun, according to a report
Who was he and why was he important?
It's hard to overstate the importance of Fakhrizadeh.
Western intelligence agencies consider him the father of Iran's nuclear weapons program, the alleged existence of which has been at the heart of Tehran's standoff with the international community for nearly two decades.
Iran has repeatedly claimed that its nuclear program has been used exclusively for peaceful purposes, but Western states accuse Tehran of trying to develop a nuclear bomb.
If that came to fruition, intelligence agencies say, it would have been Fakhrizadeh's brainchild.
In 2015, the Obama administration struck a landmark deal to curb Iran's nuclear program in exchange for sanctions relief, but it was short-lived.
In 2018, Trump withdrew from the pact and imposed some of the most severe sanctions Tehran has ever been subjected to.
A year later, Iran resumed uranium enrichment and took other steps to reactivate its nuclear program.
This alarmed western capitals, but experts believe Iran is still years away from developing a nuclear weapon.
While Fakhrizadeh was a key player in Iran's nuclear program, he was a veteran, so his experience is unlikely to have died with him.
Who did it?
Iran blamed Israel for the attack and said the operation bore the hallmarks of Israel's foreign intelligence agency, the Mossad.
While Iran has not provided evidence of Israeli involvement, Israel has not denied or claimed responsibility for Fakhrizadeh's death.
In 2018, Israel's Prime Minister Benjamin Netanyahu said that Fakhrizadeh was the head of Project Amad, which he and others describe as a secret nuclear weapons effort.
"Remember that name: Fakhrizadeh," the prime minister told reporters at the time.
Shortly after the assassination, US President Donald Trump retweeted Israeli journalist Yossi Melman.
He wrote that Fakhrizadeh "was the head of Iran's secret military program and wanted for many years by Mossad."
MIRA
: Tension between Iran and Israel after the murder of Mohsen Fakhrizadeh: some versions of state media contradict each other
What has Iran said?
The Iranians have promised retaliation, but it is unclear how or if this will materialize.
After Trump ordered the assassination of senior Iranian general Qasem Soleimani in January, Tehran raised the specter of cataclysmic retaliation.
The Iranians responded quickly, but in moderation.
In January, Iran fired several rounds of ballistic missiles at US positions in the vicinity of Iraq, but warned troops in advance, allowing them to take refuge in bunkers.
But on top of that, a military error that night led to the downing of a Ukrainian commercial jet that had taken off from Tehran's international airport, killing 176 people on board.
What does it mean for the nuclear deal?
Iran's restraint after Soleimani's assassination was seen as an integral part of President Hassan Rouhani's policy of "strategic patience."
For years, Iranians have relied on a Democrat to return to the White House.
And they are counting on US President-elect Joe Biden to deliver on his promise to restore the nuclear deal.
What happens to the nuclear deal depends on Iran's response to this assassination.
If Iran retaliates now and triggers a conflict, it would be more difficult for all parties to return to the negotiating table next year.
But Iran's patience is wearing thin.
His economy has suffered under relentless sanctions and his top leadership has taken repeated blows in recent years.
Among lawmakers, calls for retaliation are growing stronger.
And Tehran's deterrent powers have been greatly diminished by expectations of restraint.
Whether Tehran retaliates or not, it has a lot to lose and will act carefully.
Iran Nuclear Agreement
